Frustrated with Easton Berry Incorporated

I am having a problem with this company regarding an account I paid and even requested a paid up letter which I received same day of my last payment. The account was fully paid by Nov 2017, late 2018 they start calling me regarding the account I explain that it has been paid, they requested the paid up letter I emailed it then the consultant said they will get back to me which never happened. Now 2019 I get a call from another law firm saying my account has been handed over to them by Easton Berry Incorporated, I explain the same thing that the account is paid up then email them the paid up letter. They get back to me to say Easton Berry is saying the paid up letter was issued by mistake I should pay the account. Easton Berry gave me an amount and I started making monthly payments till that amount was nil so how am I suppose to pay the account again?? And why hand over my account? I am not paying that account again, I am struggling financially as it is so paying an account I know I have paid won't happen.